1. Field of the Invention
The present invention relates to an apparatus for driving an objective lens and, more particularly, to an actuator apparatus for moving an objective lens along a direction crossing a tracking guide of an optical disk.
2. Description of the Related Art
In a system for optically recording information on an optical information recording medium such as an optical disk or an opto-magnetic disk and reproducing the information from the recording medium, i.e., an optical disk system, focus servo is operated to maintain an object lens in a focusing state, a light beam is focused on an optical disk by an objective lens, and a desired tracking guide on the optical disk is accessed. In addition, tracking servo is operated to maintain the objective lens in a tracking state, and a desired track, i.e., a tracking guide or an array of tracking guides is traced by a light beam. In a state wherein the objective lens is maintained in the focusing and tracking states, a light beam is intensity-modulated to record information on the optical disk, and light which is reflected by the optical disk and returns through the objective lens is detected to reproduce information.
The optical disk system described above comprises an optical head apparatus comprising a mechanism for moving the objective lens along its optical axis, i.e., a focus direction, and along a direction perpendicular to the optical axis, i.e., a direction crossing the tracking guide so as to attain focus servo and tracking servo. The optical head apparatus is classified into a linear type apparatus for linearly moving the objective lens along a direction perpendicular to the optical axis and a rotary type apparatus for arcuately moving the objective lens along a radial direction crossing the tracking guide. The linear and rotary type apparatuses have both merits and demerits, and it cannot be determined which type is preferable. An optical head apparatus of either type is required to achieve access of the tracking guide and the tracking servo operation by the same mechanism in order to simplify a mechanism of an apparatus and to attain high-speed, high-precision access and tracking servo.
In the optical head apparatus, a lens holder for holding the objective lens using a magnetic circuit is moved in the radial and focus directions of a disk. Upon movement of the objective lens, especially, upon access of the tracking guide, a moving position of a moving body carrying the lens holder of the objective lens, i.e., the position of the objective lens in the radial direction of the optical disk must be detected. In a conventional position detection device for detecting the moving position of the objective lens, a shutter member is provided to the moving body, and is detected by detection elements such as two photointerrupters provided to a stationary base unit, thereby detecting whether or not the objective lens is located at an end of a moving range of the moving body. However, in the conventional position detection device, a detection element for detecting the shutter member when the objective lens is located at the innermost end, and a detection element for detecting the shutter member when the objective lens is located at the outermost end must be arranged adjacent to the moving body. In this apparatus, since the two detection elements are arranged to have a large gap therebetween, an optical head becomes bulky. Although the conventional position detection device can detect that the objective lens is located at the innermost or outermost end, it cannot detect whether the objective lens is located near the innermost or outermost end. Therefore, it is difficult to accurately position the objective lens at the innermost or outermost end as a home position.
In this manner, in the conventional position detection device, since a large distance between the two detection elements must be set, a device such as an optical head becomes bulky, and it cannot be detected whether or not the position of the moving body is near the end of its moving range. Therefore, it is difficult to accurately position the moving body at the end of its moving range.
The rotary type optical head apparatus is disclosed in, e.g., Published Unexamined Japanese Patent Application No. 61-123030. In the conventional rotary type optical head apparatus, a focusing magnetic circuit for generating a driving force for driving the objective lens in a focus direction and a radial magnetic circuit for generating a driving force for driving the objective lens in a radial direction are arranged in a stationary unit. In the conventional rotary type optical head apparatus, upon radial movement of the objective lens, a focusing coil must be arranged to be movable in a radial direction in the focusing magnetic circuit. For this purpose, the focusing magnetic circuit is formed to extend in the radial direction. As a result, the entire optical head becomes large in size, and since a ratio of magnetic fluxes effectively used to interlink with those of the focusing coil to magnetic fluxes generated by the focusing magnetic circuit is small, a driving force for moving the objective lens in the focus direction cannot consequently be increased. In this manner, in the conventional rotary type optical head apparatus, since the focusing magnetic circuit must be formed to extend in the radial direction, the entire optical head becomes bulky, and a ratio of magnetic fluxes effectively used of those generated by the focusing magnetic circuit is small, and a driving force for moving the objective lens along the optical axis cannot be increased.
The conventional rotary type magnetic disk apparatus is required to increase a driving force of a driving unit to attain high-speed positioning. As is generally known, in order to obtain a large driving force, a current to be flowed through a coil in the driving unit is increased to increase a Lorentz force to be generated. This method, however, requires a large driving current, resulting in an increase in power consumption. An increase in current inevitably causes an increase in heat generated from a coil, and a temperature inside the apparatus is increased. Thus, a problem of an influence of heat to other components must also be taken into consideration.
In the rotary type optical head apparatus, there is proposed a structure in which a focusing unit for supporting the objective lens to be movable along an optical axis is fixed to the outermost periphery of a rotary body which is rotated about an axis parallel to the optical axis. In general, when an almost disk-like substance is floating in a free space, it has an almost arcuated vibration mode, and the amplitudes of vibration become maximum at outermost and innermost peripheral positions. Even when a rotary body is supported by a ball bearing or the like, vibration in a specific vibration mode appears in a small apparatus such as an optical head apparatus in a frequency range of 1 kHz or higher, its vibration amplitude is almost equal to or smaller than that of a play of the bearing, and its vibration mode is substantially the same as that in a state wherein the rotary body is floating in a free space. For this reason, in an apparatus having a structure in which the focusing unit is fixed at the outermost peripheral portion, the focusing unit is largely influenced by vibration of the rotary body in a specific mode, and a highly precise control operation cannot be realized. In order to avoid the influence of vibration, the mechanical strength of the rotary body may be increased. However, a method to achieve this suffers from limitations, and a servo range required in a recording or reproduction optical head apparatus cannot be obtained.
In the conventional optical head apparatus, a base to which an optical system is fixed is formed by a aluminum die-cast to precisely mount optical components. However, in a rotary drum in which an optical system is assembled with an aluminum die-cast base, a laser diode or photodetector in the apparatus is heated by heat generated by a tracking coil, resulting in a short service life of the laser diode, and an increase in noise level of a laser beam generated from the laser diode.